There won't be a third Pokémon game released after the first two incarnations.
Pokémon Black and White was unveiled recently to the European press. Spanish site Meristation was able to talk to some of the people responsible for the fifth generation of Pokémon. When asked if there would be a Pokémon Gray after the release of Pokémon Black and White, they mentioned that they were done with Pokémon Black and White and that players shouldn't expect another game.
"No, not at all. We are done with this generation with Pokémon Black and White Edition", said Mana Ibe, designer on Pokémon Black/White.
In addition, they were asked why they released the game so close to the launch of the 3DS. Their response was "It was the right time to launch a new game with lots of innovations. While the 3DS is a great system, these games were created with the DS in mind".